Analog telemetry system having frequency signal transmission

ABSTRACT

An analog telemetry system including a plurality of analog encoders at a plurality of remote stations for sensing a plurality of different conditions and providing analog signals corresponding thereto with matching electrical characteristics. Each of the remote stations has a transmitter for transmitting the analog signals to a central control station which includes an indicating meter having a temperature scale and a graduated 0-100 scale for visually representing all of the analog signals regardless of the conditions sensed.

NR amsaasa United States Patent 1 I 3,688,292

Snively Aug. 29, 1972 [54] ANALOG TELEMETRY SYSTEM 3,194,067 7/ 1965Grillo ..340/183 X HAVING FREQUENCY SIGNAL 3,253,260 5/1966 Hawley..340/182 UX TRANSMISSION Primary Examinerl-1arold I. Pitts [72]Inventor Hugh v'smvelyRlchmondva' Attorney-Auzville Jackson, Jr., RobertL. Marben [73] Assignee: Robertshaw Controls Company, and Anthony A.OBrien Richmond, Va. [22] Filed: Feb. 16, 1970 ABSTRACT I [21] I A 11 71 An analog telemetry system including a plurality of analog encoders ata plurality of remote stations for sensing a plurality of differentconditions and provid- [52] US. Cl ..340/207 R, 340/182 R, 340/183 R inganalog Signals corresponding thereto withv [51] Int. Cl. ..Glk /00, GOlk5/ 18 matchin g e ectrical characteristics. Each of the remote [58]Field of Search ..340/ 183 R, 182 R, 207 R Stations has a transmitterfor transmitting the analog tr 1 t h l [5 References Cited signals to acen a1 contro s ation whic inc udes an indicating meter having atemperature scale and a UNITED STATES PATENTS graduated 0-100 scale forvisually representing all of the analog signals regardless of theconditions sensed.

3,145,374 8/1964 Benner et a1. .'.....340/183 3,188,394 6/1965 McMillian..340/183 X 3Claims,4Drawing Figures l4 I8 A POINT ADDRESS SELECTIONGENERATOR TRANSM'TTER 22 .6 i 56 a 64 58 1 44 5 I if 24 RECORDER H J 69r 48 62 2a LOCAL POINT 54 l ANALOG INTERFACE g L gg i gg L38 I 26 TEMPpvw -32 0 0 \ffiss '74 1 CURRENT 724 SOURCE I 34 401 LOCAL POINTS lANALOG TELEMETRY SYSTEM HAVING FREQUENCY SIGNAL TRANSMISSION BACKGROUNDOF THE INVENTION 1. Field of the Invention The present inventionpertains to analog telemetry systems and more particularly to suchsystems utilizing a single indicating meter.

2. Description of the Prior Art An operator at the central controlstation of a supervisory control system for a plurality of remotestations normally must view a plurality of indicating meters corresponding to all of the various conditions being monitored from thecentral control station. The use of a plurality of such meters is aninconvenience to the operator in that it is difficult to become familiarwith and'accurately obtain information from a plurality of meters, eachof which has a different scale but the same general appearance.

One of the reasons for utilizing an indicating meter for each conditionsensed in the field inconventional supervisory control systems is thatthe analog signals sensed in the field have varying ranges associatedtherewith, thereby requiring either range selection signals to betransmitted back to the central control station from the remote stationsor separate wires or frequency channels for various conditions. Whenvoice-grade telephone lines are utilized as a medium of communicationbetween the remote stations and the central control station, the numberof available frequency channels is limited; and, accordingly, it isdisadvantageous to use a plurality of the frequency channels for analogsignals. Similarly, the cost of wiring is expensive; and, therefore, theuse of separate wires for each analog condition sensed isdisadvantageous.

SUMMARY OF THE INVENTION Accordingly, it is an object of the presentinvention to utilize a single indicating meter to represent analogsignals corresponding to conditions sensed at a plurality of remotestations.

The present invention is summarized in an analog telemetry systemincluding a plurality of remote stations, each of the remote stationsincluding first and second analog encoders generating first and secondanalog signals having matching electrical characteristics correspondingto first and second conditions, respectively, and a transmitter fortransmitting the first and second analog signals to a central controlstation, the central control station including a single indicating meterto visually represent the first and second conditions in accordance withthe first and second analog signals.

Another object of the present invention is to match the electricalcharacteristics of analog signals representing various conditions atremote stations prior to the communication of the analog signals to anindicating meter at a central control station.

A further object of the present invention is to utilize a singlefrequency channel to transmit analog signals representing variousconditions to a central control stanon.

Some of the advantages of the present invention over prior art analogtelemetry systems are that the number of frequency channels required foranalog operations are reduced, a single-meter represents all fieldconditions, and circuitry for analog monitoring is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of ananalogtelemetry system according to the present invention.

FIG. 2 is a schematic diagram of the interfaces and field points of FIG.1.

FIG. 3 is a schematic diagram of analog encoder circuitry at the localpoints of FIG. 1.

FIG. 4 is a schematic diagram of analog encoder circuitry at the remotepoints of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT An analog telemetry systemaccording to the present invention is illustrated in FIG. 1 and includesa central control station 10 and a plurality of remote stations RS1 andRS2. The remote stations are in communication with central controlstation 10 through a voicegrade telephone line 12 in a party-line mannersuch that each remote station receives signals from central controlstation 10, simultaneously. While only two remote stations areillustrated in FIG. 1, any number of remote stations may be utilizedwith the analog telemetry system of the present invention. Similarly,various means of communication may be utilized with the analog telemetrysystem of the present invention; for instance, individual voice-gradetelephone lines may be utilized for each remote station with a pair ofcontacts selectively controlling communication between the remotestations and central control station 10. Similarly, any suitablecommunication medium such as electromagnetic transmission, may beutilized with the analog telemetry system of the present invention.

Central control station 10 includes point selection I circuitry 14 forselecting a field point to be monitored.

Point selection circuitry 14 is conventional and may include thumbwheels, toggle switches or the like. An output 16 from point selectioncircuitry 14 is supplied to an address generator 18 which provides fieldpoint signals at an output 20 to a frequency shift transmitter 22.Transmitter 22 has an output 24 supplying mark, space and carrierfrequencies corresponding to the address signals to telephone line 12for communication to the remote stations.

A frequency shift analog telemetry receiver 26 receives frequency shiftsignals from the remote stations corresponding to analog signalsgenerated thereat, and supplies the analog signals to output leads 28and 30. An output 32 from analog telemetry receiver 26 supplies a signalto a group of local field points 34 when a carrier frequency is receivedby analog telemetry receiver 26. Address signals are supplied to localpoints 34 from output 20 of address generator 18 via an output 36 of alocal point interface 38. Analog encoder circuitry in local points 34supplies analog signals to output leads 40 and 42 which are con nectedwith output leads 28 and 30 from analog telemetry receiver 26,respectively. The analog outputs from analog telemetry receiver 26 andlocal points 34 are supplied to a record relay 44 and an indicate relay46 for selective communication by a single-pole, double-throw manualselector switch 48 to a recorder 50 or an indicating meter 52.

Record relay 44 includes a coil 54 connected between a source ofnegative potential and a contact of selector switch 48. A pair ofcontacts 56 are connected between output leads 28 and 40 and an input 58to recorder 50. Another pair of contacts 60 are connected between outputleads 30 and 42 and an input 62 to recorder 50. The analog signals fromanalog telemetry receiver 26 and local points 34 are supplied torecorder 50 on inputs 58 and 62 when record relay 44 is energized, andrecorder 50 also receives an input on a lead 64 from address generator18 indicating the field point being interrogated. Field point addressesmay be stored by recorder 50 in program fashion and supplied to addressgenerator 18 on a lead'65 to provide automatic interrogation ofpredetermined field points.

Indicate relay 46 includes a coil 66 connected between the source ofnegative potential and another contact of selector switch 48. Asingle-pole, double-- throw switch 68 responsive to coil 66 has onecontact connected with output leads 28 and 46, the other contactconnected with an output 70 from a current source 72, and the poleconnected with an input 74 to meter 52. A single-pole, double-throwswitch 76 also responsive to coil 66 has one contact connected withoutput leads 30 and 42, the other contact connected with an output 78from current source 72, and the pole connected with an input 80 to meter52. The analog signals from analog telemetry receiver 26 and localpoints 34 are supplied to meter 52 on inputs 74 and 86 when indicaterelay 46 is energized, and a constant current from current source 72 issupplied to meter 52 on inputs 74 and 80 when indicate relay 46 isdeenergized.

Selector switch 48 is manually operable to exclusively supply the analogsignals from analog telemetry receiver 26 and local points 34 to eitherrecorder 50 or meter 52 by selectively connecting a source of positivepotential to coils 54 and 66 to energize either record relay 44 orindicate relay 46. Of course, if it is desired, the selector switch maybe designed to supply the analog signals to both recorder 50 and meter52 simultaneously.

Meter 52 has a temperature scale, preferably with a range of 40 F. to+260 F., and a graduated -100scale. A single pointer P is utilized as anindicator for both scales and is responsive to small current signals oninputs 74 and 80 to represent an analog value.

The remote stations are essentially identical and accordingly onlyremote station RS1 is described. Remote station RS1 includes a receiver82 which receives frequency shift signals corresponding to field pointaddresses from transmitter 22 at central control station 10, andreceiver 82 has an output 84 supplying the address signals to aninterface 86. Interface 86 supplies outputs to a group of remote points88 on multiconductor cables 90 and 91. Remote points 88 include analogencoder circuitry providing analog signals to an analog telemetrytransmitter 92 on leads 94 and 96, and the analog encoder circuitryfurther controls the operation of analog telemetry transmitter 92through 98 and 100. Analog telemetry transmitter 92 provides frequencyshift signals on an output 102 corresponding to the analog signals onleads 94 and 96 for communication to analog telemetry receiver 26 atcentral control station over telephone line 12.

Interface 86 and remote points 88 are illustrated schematically in FIG.2 with the realization that the circuitry therein is also utilized forlocal point interface 38 and local points 34 at the central controlstation 10. Interface 86 receives address signals from receiver 82 onoutput 84, and the address signals are supplied to a remote stationdecoder 164 and an AND gate 106 which also receives an input from remotestation decoder 104. Gate 106 is inhibited until the address of theremote station with which the interface is associated is received fromcentral control station 10 at which time gate 106 is enabled to pass theaddress signals on to registers 108 and 110, each of which has tenoutputs indicated generally at 112 and 114 corresponding to units andtens addresses of individual remote points in group 88, respectively.

The remote points include a plurality of coils arranged in a ten by tenmatrix for receiving the outputs 112 and 114 from registers 108 and 110on cables 90 and 91, respectively. If a point at remote station RS1having an address of 73 is selected, that address will be presented toregisters 168 and 116 in such a manner that the output at 112 will placea positive potential on the units lead corresponding to 3 and the outputat 114 will place a negative on the tens lead corresponding to 76. Thus,it may be seen that a complete current path can be traced from unitslead 3 through the coil indicated at 73 to tens lead 70 therebyenergizing coil 73. Each of the coils will have one or more sets ofcontacts associated therewith such that various conditions of an alarmor analog nature as well as control functions may be monitored at theremote stations.

While the present invention pertains to analog telemetry systems, itshould be realized that such systems will normally be included incomprehensive supervisory control systems capable of performing alarmand status monitoring functions and control functions as well as analogtelemetering. Accordingly, any suitable technique may be utilized toaddress individual field points; and, the skeletal circuitry of FIG. 2is shown and described only for purposes of completeness.

Analog encoder circuitry for local points 34 is illustrated in FIG. 3. Atemperature encoder 116 includes a coil 118 corresponding to one of thematrix coils illustrated in FIG. 2 and normally open contacts 120, 122and 124. Contacts are connected between a source of positive potentialand a wire 126 which is included in a trunk 128 along with wires 130 and132. Wires 130 and 132 are connected with a thermocouple 134 throughcontacts 122 and 124, respectively. Trunk 128 extends to allthermocouple temperature sensing local points such that when any of thethermocouple, temperature sensing local points are addressed, a positivepotential will be placed on wire 126 and analog voltage signals will beplace on leads 130 and 132 by thermocouple 134. A relay 1136 has a coil138 connected between wire 126 and a source of negative potential,normally open contacts 140 and 142, and normally closed contacts 144 and146. The analog signals on wires 136 and 132 are supplied to a convertor148 which supplies the analog signals after conversion to a common formto output leads 40 and 42 through contacts 140 and 142, respectively,when relay 136 is energized. Contacts 144 and 146 are connected withoutput leads 46 and 42 through normally closed contacts 149 and 150which are controlled by a coil 152 which is energized by a positivepotential on lead 32 when analog telemetry receiver 26 receives acarrier signal from any of the analog telemetry transmitters 92 attheremote stations.

A humidity encoder 154 includes a coil 156 corresponding to one of thematrix coils illustrated in FIG. 2 and normally open contacts 158, 160and 162. Contacts 158 are connected between a source of positivepotential and a wire 164 which is included in a trunk 166 along withwires 168 and 170. Wires 168 and 170 are connected with a humiditysensor 172 through contacts 160 and 162, respectively. Trunk 166 extendsto all humidity sensing local points such that when any of the humiditysensing local points are addressed, a positive potential will be placedon wire 164 and analog current or voltage signals will be placed onwires 168 and 170 by humidity sensor 172. A relay 174 has a coil 176connected between wire 164 and the source of negative potential,normally open contacts 178 and 180, and normally closed contacts 182 and184. The analog signals on wires 168 and 170 are supplied to a convertor186 which supplies the analog signals after conversion to a common formto output leads 40 and 42 through contacts 178 and 180 when relay 174 isenergized. Contacts 182 and 184 are connected in series with contacts146 and 144, respectively, of relay 136.

A process variable encoder 188 includes a coil 190 corresponding to oneof the matrix coils illustrated in FIG. 2 and normally open contacts192, 194 and 196. Contacts 192 are connected between a source ofpositive potential and a wire 198 which is included in a trunk 200 alongwith wires 202 and 204. Wires 202 and 204 are connected with a processvariable sensor 206 through contacts 194 and 196, respectively. Trunk200 extends to all process variable local points such that when any ofthe process variable local points are addressed, a positive potentialwill be placed on wire 198 and analog current or voltage signals will beplaced on wires 202 and 204 by process variable sensor 206. A relay 208has a coil 210 connected between wire 198 and the source of negativepotential, normally open contacts 212 and 214, and normally closedcontacts 216 and 218. The analog signals on wires 202 and 204 aresupplied to output leads 40 and 42 through contacts 212 and 214 whenrelay 208 is energized. Contacts 216 and 218 are connected in serieswith contacts 184 and 182, respectively, of relay 174.

A feedback encoder 220 includes a coil 222 corresponding to one of thematrix coils illustrated in FIG. 2 and normally open contacts 224, 226and 228. Contacts 224 are connected between a source of positivepotential and a wire 230 which is included in a trunk 232 along withwires 234 and 236. Wire 234 is connected with a slider 238 throughcontacts 226, and wire 236 is connected with one end of a potentiometer240 through contacts 228. Slider 238 is movable on potentiometer 240 inaccordance with movement of a control device to provide an analogfeedback signal. Trunk 232 extends to all feedback local points suchthat when any of the feedback local points are addressed, a positivepotential will be placed on wire 230 and analog voltage signals will beplaced on wires 234 and 236 by slider 238. A relay 242 has a coil 248connected between wire 230 and the source of negative potential,

normally open contacts 250 and 252, and normally closed contacts 254 and256. The analog signals on wires 234 and 236 are supplied to a convertor258 which supplies the analog signals after conversion to a common formto output leads 40 and 42 through contacts 250 and 252 when relay 242 isenergized. Contacts 254 and 256 are connected in series between theoutputs of a constant current source 260 and contacts 218 and 216,respectively, of relay 208.

Analog encoder circuitry for remote points 88 is illustrated in FIG. 4and is similar to the analog encoder circuitry of FIG. 3. Accordingly,the components of FIG. 4 which are identical to components of FIG. 3 aregiven identical reference numbers with primes and are not describedagain.

Relay 136' for temperature encoder 116' includes normally open contacts262 which are connected between a source of positive potential and acoil 264 in a relay 266. Relay 266 includes normally open contacts 268which are connected with analog telemetry trans mitter 92 through leads98 and 100. Relay 174' includes normally open contacts 270 connectedbetween the source of positive potential and coil 264, and relay 208'includes normally open contacts 272 connected between the source ofpositive potential and coil 264. Similarly, relay 242' includes normallyopen contacts 274 connected between the source and positive potentialand coil 264.

When any remote point corresponding to an analog encoder in group 88 isaddressed, the relay associated with the condition being sensed will beenergized to in turn energize coil 264 of relay 266 through either ofcontacts 262, 270, 272 or 274. When relay 266 is energized, the closureof contacts 268 keys analog telemetry transmitter 92 to provide acarrier frequency prior to the transmitting of analog signals on leads94 and 96.

In operation, if no field point has been addressed at central controlstation 10, the analog telemetry transmitters 92 at the remote stationswill be passive. Accordingly, analog telemetry receiver 26 will receiveno input signals, and coil 152 which is responsive to output 32 ofanalog telemetry receiver 26 will be deenergized. Since no local pointshave been addressed, relays 136, 174, 208 and 242 will also bedeenergized, and a path for current from current source 260 is completedto record relay 44 and indicate relay 46.

lf selector switch 48 is in the record position illustrated in FIG. 1,record relay 44 will be energized to close contacts 56 and to permitcurrent flow to recorder 50. Indicate relay 46 will be deenergized atthis time; and, accordingly, switches 68 and 76 will be in a position toprovide current from source 72 to meter 52 to assure the continuoussupply of current required by the meter. lf selector switch 48 is placedin the indicate position, indicate relay 46 will be energized to permitcurrent from current source 260 to be supplied to meter 52 throughswitches 68 and 76 to provide continuous current to the meter.

When it is desired to monitor a field point, point selection circuit 14or recorder 50 is operated to provide address, generator 18 withinformation corresponding to the field point and the remote station atwhich it is located. Address signals corresponding to the selected fieldpoint are communicated from the transmitter 22 to all of the remotestations and directly from output 26 of address generator 18 to localpoint interface 38 simultaneously.

When the decoder we at interface 86 or local point interface 38 isactuated, gate 166 is enabled to supply address signals corresponding tothe individual field points to registers 108 and ill). The outputs 112and M4 from the registers select the desired field point in the matrixto enable the analog encoder associated therewith.

If the desired field point is located within local points 34, one of therelays 136, 174, 208 or 242 will be energized to interrupt current flowfrom current source 260. Assuming that coil 118 of temperature encoder116 has been energized in the matrix the closure of contacts 120 willenergize coil 138 to close contacts 146 and 142 and open contacts M4 and146. A small analog voltage from thermocouple 134 will be supplied onwires 130 and 1132 to convertor 148. Convertor 1148 converts the analogvoltage signal from thermocouple 134 to an analog current signalcompatible with meter 52 and having the same electrical characteristicsas analog signals from the other analog encoders. The analog currentsignals are supplied to meter 52 through contacts T40 and M2, leads 46and 42 and switches 68 and 76 if indicate relay 46 is energized. Thetemperature sensed by thermocouple 134 will be visually indicated to theoperator at central control station 10 by movement of pointer P over thetemperature scale. lf record relay 44 is energized, the analog currentsignals will be recorded at recorder 50 along with the field pointaddress on output 64 from address generator 18.

If the selected field point corresponds to coil 222 of feedback encoder220 at local points 34, the voltage across potentiometer 240 will besupplied to convertor 258 on wires 234 and 236. Convertor 258 willconvert the analog voltage signals from potentiometer 240 into analogcurrent signals compatible with meter 52 and having the same electricalcharacteristics as the analog signals from the other analog encoders Theanalog signals from feedback encoder 220 are supplied to meter 52 whenindicate relay 46 is energized, and the movement of the control devicewhich controls the position of slider 238 on potentiometer 240 will bevisually indicated by movement of pointer P over the graduated -100scale.

In a similar manner, humidity conditions and process variable conditionssuch as pressure or flow are supplied to either recorder 50 or meter 52.The analog signals from process variable sensor 206 are applied directlyto meter 52 without conversion in order to illustrate that the systemmay be designed around a specific set of analog signals thereby reducingthe equipment required; that is, the common form and electricalcharacteristics of the analog signals are defined by the analog signalsfrom process variable encoder 188.

The operation of analog encoders at remote points in groups 88 at theremote stations is similar to that described with respect to the analogencoders at local points 34. However, when either of relays H36, 174',208' or 242' are energized, coil 264 of relay 266 is energized to closecontacts 268 and cause analog telemetry transmitter 92 to transmit acarrier frequency to analog telemetry receiver 26 at the central controlstation. Upon receiving the carrier frequency, analog telemetry receiver26 energizes relay 52 to interrupt the current from current source 260.The analog signals on leads 94 and 96 all have a common form and matchedelectrical characteristics and are supplied to analog telemetrytransmitter 92 to cause the transmitting of mark and space signalshaving a frequency of alternation corresponding to the analog signals.The mark and space alternations are received at analog telemetryreceiver 26 which supplies analog signals on output leads 2% and 30corresponding to the analog signals on leads 9 1 and 96 from fieldpoints at the remote stations. 2

By converting the analog signals at the encoders to a common form withthe same electrical characteristics,

a single frequency channel can be utilized to transmit the analogsignals from the remote stations to the central control stations and asingle meter can be utilized to visually represent conditions sensed atthe field points. Thus, the analog telemetry system of the presentinvention permits greater use of the limited frequency channelsavailable with communication mediums such as telephone lines.

Meter 52 may, for example, receive analog current signals in anoperating range from 4-20 ma; and, ac-

cordingly, the convertors will provide analog signal outputs havingelectrical characteristics compatible with meter 52. That is, theconvertors will provide analog current signals within the 4-20 ma range.For instance, potentiometer 240 may have a maximum voltage of 24 v. andthe analog signals on wires 234 and 236 will have -a range of 0-20 v.Convertor 258 will convert the analog voltage signals to a 4-20 ma.range to thereby match the electrical characteristics of the analogsignals from the other analog encoders.

For illustrative purposes examples of equipment and signal ranges areprovided for each of the different field points shown, as used toprovide 4-20 ma analog current signals. Thermocouple 134 may becopper-constantan and provide a 3 to 7 mv. output for a 350 F. range,and convertor 148 may be a Robertshaw Series 12 TransmittingPotentiometer which converts the 3 to +7 mv. signal to the 4-20 maanalog range. Humidity sensor 172 may provide a pneumatic pressure from3 to 15 lbs. which is supplied to a Penn Controls P-.80 transducer toprovide a resistance between 0-135 ohms in accordance with the pressure,and convertor 186 may be a Rochestor Instrument Model SC370 whichutilizes a voltage to sense the resistance of the transducer andprovides an output corresponding thereto in the 4-20 ma analog rangeProcess variable sensor 206 may be a Robertshaw Model which sensespressure and provides an output in the 4 to 20 ma range. The convertor258 for potentiometer 246 may be a Rochestor Instrument Model SC300Xwhich receives a 0-20 v. input from the potentiometer and provides anoutput corresponding thereto in the 4-20 ma range.

An example of a commercially, available, single meter that may beutilized for meter 52 of the present invention is the Westronics Model 1l. Analog telemetry receiver 26 and analog telemetry transmitters 92 arecommercially available as Models QATR-ZO and QATT-ZO, respectively,manufactured by Quindar Electronics.

Inasmuch as the present invention is subject to many variations,modifications and changes in detail, it is intended that all mattercontained in the foregoing description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

1. An analog telemetry system comprising a plurality of remote stations,each of said remote stations comprising a plurality of analog encodermeans each sensing a separate condition and generating an analog signalcorresponding to said sensed condition,

each analog encoder means having conversion means for converting theelectrical characteristics of each said analog signal to match theelectrical characteristics of the other analog signals and lying withina predetermined analog signal range,

means adapted to selectively enable one of said analog encoder means,and

transmitter means connected with each of said analog encoder means totransmit signals from said analog encoder means having said matchedelectrical characteristics and lying within said analog signal rangeover a single frequency channel; and a central control stationcommunicating with said plurality of remote stations, said centralcontrol station comprising means to select one of said analog encodermeans at one of said plurality of remote stations, receiver means forreceiving said analog signals from said transmitter means over saidsingle frequency channel, and a single indicating meter means connectedwith said receiver means and having an operating range corresponding tosaid predetermined analog signal range to visually represent any of saidsensed conditions in accordance with said received analog signal.

2. The invention as recited in claim 1 wherein said central controlstation further comprises current source means, switch means connectingsaid current source means with said meter, and means in said receivermeans responsive to transmission of said analog signals by saidtransmitter means to open said switch means to interrupt the supply ofcurrent from said current source means to said meter.

3. The invention as recited in claim 1 wherein said central controlstation further comprises a plurality of local points having analogencoder means each sensing a separate local condition and generating ananalog signal corresponding to said sensed condition, each local analogencoder means having electrical charac teristics matching the electricalcharacteristics of said remote analog signals, said local analog encodermeans having output means connected with said meter to enable visualrepresentation of said local conditions in accordance with said localanalog signals.

1. An analog telemetry system comprising a plurality of remote stations,each of said remote stations comprising a plurality of analog encodermeans each sensing a separate condition and generating an analog signalcorresponding to said sensed condition, each analog encoder means havingconversion means for converting the electrical characteristics of eachsaid analog signal to match the electrical characteristics of the otheranalog signals and lying within a predetermined analog signal range,means adapted to selectively enable one of said analog encoder means,and transmitter means connected with each of said analog encoder meansto transmit signals from said analog encoder means having said matchedelectrical characteristics and lying within said analog signal rangeover a single frequency channel; and a central control stationcommunicating with said plurality of remote stations, said centralcontrol station comprising means to select one of said analog encodermeans at one of said plurality of remote stations, receiver means forreceiving said analog signals from said transmitter means over saidsingle frequency channel, and a single indicating meter means connectedwith said receiver means and having an operating range corresponding tosaid predetermined analog signal range to visually represent any of saidsensed conditions in accordance with said received analog signal.
 2. Theinvention as recited in claim 1 wherein said central control stationfurther comprises current source means, switch means connecting saidcurrent source means with said meter, and means in said receiver meansresponsive to transmission of said analog signals by said transmittermeans to open said switch means to interrupt the supply of current fromsaid current source means to said meter.
 3. The invention as recited inclaim 1 wherein said central control station further comprises aplurality of local points having analog encoder means each sensing aseparate local condition and generating an analog signal correspondingto said sensed condition, each local analog encoder means havingelectrical characteristics matching the electrical characteristics ofsaid remote analog signals, said local analog encoder means havingoutput means connected with said meter to enable visual representationof said local conditions in accordance with said local analog signals.